Twin-belt casting machines have been used for casting metals for quite some time. In machines of this kind, endless belts rotating in race-track patterns are positioned one above the other (or, in some cases, side-by-side) with generally planar parallel runs of each belt positioned closely adjacent to each other to define a mold therebetween. Molten metal is introduced into the mold at one end and the metal is drawn through the mold by the moving belt surfaces. Heat from the molten metal is transferred through the belts, and this transfer is assisted by cooling means, such as water sprays, acting on the opposite sides of the belts in the regions of the mold. In consequence, the metal solidifies as it passes through the mold, and a solid metal slab or strip emerges from the opposite end of the mold. For example, improved casting machines of this kind are described in U.S. Pat. Nos. 4,008,750 and 4,061,177 issued respectively on Feb. 22, 1977 and Dec. 6, 1977 to the same assignee as the present application. The casting machines also use high efficiency coolant application systems such as are described in U.S. Pat. No. 4,193,440 issued on Mar. 18, 1980 to the same assignee as the present application and in International Application Publication WO 02/11922 filed on Aug. 7, 2001 also by the same assignee as the present application. The disclosures of all these publications are incorporated herein by reference.
These casting machines, with their high efficiency coolant application systems, operate by creating a thin, high velocity stream of coolant behind the casting belt. This results in a high maximum heat transfer coefficient between coolant and belt. The belt in addition “floats” on the coolant layer in the critical areas of the casting, rather than merely being supported between pulleys.
The belts used in casting machines of this kind are usually made of textured steel or, less commonly, of copper. Such materials are disclosed in, for example, U.S. Pat. No. 5,636,681 issued on Jun. 10, 1997 to the same assignee as the present application. Furthermore, U.S. Pat. No. 4,915,158 issued on Apr. 10, 1990 and assigned to Hazelett Strip-Casting Corporation discloses a copper belt providing a backing for a ceramic coating. However, belts made of these materials (particularly those made of copper) are expensive to manufacture and copper belts are susceptible to “plastic set” (i.e. distortion due to handling or lack of external support systems). Moreover, steel belts tend to have thermal conductivities that are suitable only for casting non-ferrous and light metal alloys of one kind, whereas copper belts have thermal conductivities suitable for non-ferrous and light metal alloys of another kind. For example, textured (e.g. shot-blasted) steel belts may be used for many relatively short freezing range aluminum alloys, such as fin or foil alloys, whereas copper belts are required for surface critical applications, e.g. for automotive aluminum alloys having longer freezing ranges than normal. A process for casting such automotive alloys using the high heat flux capability of copper belts is disclosed in U.S. Pat. No. 5,616,189 issued on Apr. 1, 1997 to the same assignee as the present application. In that reference, heat fluxes as high as 4.5 MW/m2 are found suitable, and such heat fluxes normally require the use of Cu belts. Other long freezing range alloys, for example those described in Leone et al., Alcan Belt Casting Mini-Mill Process, May 1989, are preferably cast at even higher heat fluxes (over 5 MW/m2).
However, due to the higher thermal conductivity of copper belts, such belts cannot be used to cast light gauge alloys due to the onset of a casting defect referred to as “shell distortion” (caused by a variation in ingot cross-section resulting from regions of higher heat transfer formed adjacent to low heat transfer regions, i.e. uneven heat removal). Consequently, when the casting apparatus is used for casting a variety of non-ferrous metal alloys, it is frequently necessary to change the belts from steel to copper or vice versa between casting operations. This is time consuming, expensive and troublesome. In modern casters of the type described above, it is desired as well that they operate at a wide range of throughput, also requiring easy operation at high heat fluxes.
Moreover, Applicants have found that textured steel belts require the use of a different parting agent application system than copper belts (brushes versus rotating atomizing bells and a cleaning box), so that it is necessary to change the parting agent application system when changing alloy systems. U.S. Pat. No. 3,414,043 issued on Dec. 3, 1968 to A. R. Wagner, discloses a casting process in which a mold is formed between advancing single-use strips. The strips are made of the same material as the molten metal (which is not identified), but strip material may be incorporated into the final product, which is obviously not acceptable for belt casters.
There is therefore a need for improvements in the belts used in belt casting machines of the type described above.